Japanese Patent Application Laid-Open No. 5-110941 discloses a prior art. This prior art is characterized in having an output control circuit which reads out output data for each field from a photosensitive section of the CCD image pickup device through a transfer section, and reads out again output data for one line of the photosensitive section, synchronously with the first horizontal synchronizing signal after readout of the output data for each field, so that a level difference in black level can be corrected by a black level correction signal obtained for each field scan, without requiring manual adjustment.
Japanese Patent Application Laid-Open No. 2000-152098 discloses another prior art. This prior art comprises an OB level calculation unit of a picture signal obtained from an imager, a coefficient calculation unit and a subtraction circuit are provided. An OB level difference signal is generated by multiplying the output from the OB level calculation unit by a coefficient in the coefficient calculation unit, and the OB level difference signal is subtracted from the picture signal, using the subtraction circuit, to thereby correct the OB level difference.
FIG. 2 is a graph which shows an example of a conventional curve showing a relationship (b) between an exposure time (t) of a normal CCD image pickup device and an output luminance level (y).
Recently, small size, high performance and high function, and high speed have been achieved in the world of digital cameras. On the other hand, from the viewpoint of power consumption, it cannot be said that power consumption is clearly increasing, but neither be said that it is decreasing. As described above, power consumed by the equipments does not change. On the other hand, as small size and high speed have been achieved, temperature in the equipment increases, to thereby cause various problems.
Among these problems, characteristic deterioration due to heat is intense in the CCD image pickup device which has been widely used as an imager in a digital camera. When the characteristic is deteriorated due to heat, there are such problems that a significant point appears due to an increase of dark current in a unit of pixel, which is referred to as temperature flaw, quantity of saturated signals decreases, and dark current on the whole image increases. Among these, the increase of dark current on the whole image means that the increase is constructionally different between an OB (Optical Black) section, and an effective pixel section. This is because the OB section is covered with an aluminum shading film, and hence parasitic capacitance is generated between a photo diode and the aluminum shading film, thereby the surface level thereof becomes lower than that of the effective pixel section. This phenomenon is generally referred to as “OB level difference”.
Therefore, the dark current component generally increases in the effective pixel section, but when the CCD signal is sampled by CDS (Correlated Double Sampling) in the subsequent stage, clamping is performed by designating the output level in the OB section as the black level. Hence, if the black level is different between the OB section and the effective pixel section, such a problem occurs that even if in a shaded state, the screen becomes bright as if light shines thereon.
In equipments which take motion pictures, such as video cameras, or in digital still cameras, under a so-called monitoring state, since the frame rate is constant at the time of low illuminance when the incident ray volume is small, the signal quantity of the CCD cannot be increases by making the exposure time long as in a camera. Hence, the CCD output is amplified in an amplifier in the subsequent stage. The level thereof is about 30 dB.
For example, if a gain of as large as 30 dB is multiplied under a condition that the OB level difference is 5 mV, only the dark current due to the OB level difference reaches a level of as large as 160 mV. This means that the reference signal level to be obtained has been reached only by the dark current. Thus, if the OB level difference is not corrected, considerable deterioration occurs in the image quality.
As explained above, the OB level difference means a variation in pedestal level due to heat, in other words, an offset quantity caused by sampling of the CCD output signal by the CDS.